semi合集-English.pdf - 第3761页
SEMI F5-1101 © SEMI 1990 , 2001 5 8 End-of-Pipe A batement Technologies 8.1 Group 1 — Acid Abatement 8.1.1 T he appropriate a batement technology for this category is comm only called " wet scrubbing." 8.1.2 Ma…
SEMI F5-1101 © SEMI 1990, 2001 4
7.10 Many types of process equipment use and exhaust
incompatible gases sequentially (e.g., silane and then
nitrogen trifluoride (NF
3
) as elements of a deposition
and cleaning cycle).
7.10.1 With more than one of these processes being
discharged asynchronously into a common exhaust
duct, undesired reactions may occur.
7.10.2 It is possible to avoid this problem by fitting the
equipment outlet with three-way valves to divert the
discharge to the correct exhaust system, depending on
equipment process step. Three-way valves may not be
necessary if pump ballast-gas flow is increased to dilute
gases.
7.10.2.1 Switching must accurately account for the gas
transmission times from process chamber inlet to the
exhaust valve.
7.10.2.2 Introduction of additional mechanical devices
could create major hazards, reliability, and service or
maintenance issues.
7.10.2.3 Valves must incorporate failsafe designs to
ensure pump outlet is not shut off.
7.10.3 An alternative solution is to fit a local POU
abatement system that treats at least one component and
is not adversely affected by the other components.
7.11 Point-of-Use (POU) Abatement Devices
7.11.1 POU abatement devices are designed to operate
at the process tool or pump outlet, and to remove
hazardous gases BEFORE they enter the exhaust
ductwork.
7.11.2 An end-of-pipe scrubber does not protect the
facility from reactions in the ductwork. Reactions
between process gases, as discussed above, or reactions
with other process by-products within the ductwork
itself are not prevented by end-of-pipe systems either.
7.11.3 POU abatement devices may be appropriate
wherever the exhaust gases can react in the ductwork.
7.11.4 If process systems are not treated with POU
abatement devices, the higher concentrations of
hazardous gases will present a higher risk during a leak
into the facility space.
7.11.5 Improperly functioning POU abatement devices
can create backpressure that can increase the risk of a
leak.
7.12 Four significant consequences can be foreseen
from effluent gas reactions in the ductwork:
7.12.1 Blocked Ducts — Reactions between gases, or
condensation of vapors, can produce solids and semi-
solids (such as crystals, slurries or gels) that will block
the ductwork.
7.12.1.1 Collection of solids can create dead leg
sections that may contain reactive byproducts or unused
raw materials. Catastrophic release of these trapped
materials could generate a significant hazard.
7.12.1.2 Extensive downtime (for clean out, or ducts
collapsing under the weight of accumulated solids) is a
possible undesirable outcome of this condition.
7.12.1.3 Solids deposited (such as from metal etching)
can, themselves, be both corrosive and hazardous,
leading to health risks for people performing cleaning
in ductwork. (See Related Documents 13.1 through
13.3).
7.12.1.4 Blocked ducts may result in toxic process
gases or by products escaping into the work area.
7.12.2 Duct Corrosion — Etching and some chamber
cleaning process emissions are corrosive. Both metallic
(including stainless steel) and plastic ducting can be
attacked, resulting in the release of hazardous gas or
condensed liquids into the workplace.
7.12.3 Duct Fires or Explosions — Flammable and
pyrophoric gases can ignite in the ductwork. If the
ductwork is combustible or has flammable or
combustible deposits in it (e.g., hydrocarbon pump oil),
facility fires can result.
7.12.4 Additional information on protection of
Industrial Exhaust systems from fires may be found in
FM Global Loss Prevention Data Sheet 7-78,
“Industrial Exhaust Systems”.
NOTE 4: Destruction of whole production lines with multi-
million dollar cost impact has resulted from such incidents.
7.12.5 Formation of Ammonium Compounds — If
ammonia is discharged into a duct with acid
compounds, a sub-micron ammonium compound fume
can be generated, that is not easily removed by end-of-
pipe systems.
7.12.5.1 The result is visible discharges.
7.12.5.2 Ammonium compounds produce solids in the
duct.
7.13 If any of the above reactions can occur in the
process exhaust, POU abatement should be considered.
7.14 Exhausts of oil-lubricated vacuum pumps should
be fitted with well-maintained oil mist separators/
filters.
7.15 POU abatement devices may also be needed
upstream of the recovery device for the recovery of
some gaseous compounds (e.g., hydrogen (H
2
), PFC
gases).
SEMI F5-1101 © SEMI 1990, 20015
8 End-of-Pipe Abatement Technologies
8.1 Group 1 — Acid Abatement
8.1.1 The appropriate abatement technology for this
category is commonly called " wet scrubbing."
8.1.2 Many, if not all, of the semiconductor processes
have traditionally been exhausted through centralized
wet scrubbers.
8.1.2.1 This procedure has limited technical
acceptability and may not meet the requirements of all
regulatory agencies.
8.1.2.2 This technology should be targeted at
abatement of acid gases, vapors, mists, and fumes
emitted from CVD, dry etching, and wet chemical
operations.
8.1.2.3 Most wet chemical processes are typically
carried out under local exhaust ventilation devices, such
as fume hoods, or in specially designed rooms that are
totally exhausted. These types of operations include:
• Processes used to clean wafers or clean wafer
handling equipment (such as quartz components,
tubing),
• Processes used in wafer fabrication for surface
treatment such as oxide, silicon nitride and other
surface treatments, (e.g., metal etching), and
• Processes used for development of positive
photoresist.
8.1.2.4 The chemical species emitted depends on both
the chemicals used in the processes and the conditions
of use.
8.1.2.5 Volatile acids (e.g., hydrochloric (HCl),
hydrofluoric, nitric, and acetic) yield corresponding
gases and vapors, especially when heated.
8.1.2.5.1 The reaction of hydrochloric and nitric acids
in aqua regia yields nitrosyl chloride (NOCl) vapor and
elemental chlorine (Cl
2
) gas.
8.1.2.5.2 When sulfuric acid is heated, it yields both
sulfuric acid vapor (H
2
SO
4
) and sulfur trioxide (SO
3
).
8.1.2.5.3 Phosphoric acid (H
3
PO
4
) is not significantly
volatile, but spray may be carried over into the exhaust
system by mechanical entrainment.
8.1.2.5.4 Under appropriate conditions, spray droplets
of any of the chemicals used may be mechanically
entrained into the ventilation air exhausted from the
various processes.
8.1.3 Design and selection of scrubbing equipment for
the acid exhaust systems must take into account both
the chemical and physical properties of the materials
being emitted.
8.1.4 The principles involved in collection of
particulate matter (including liquid aerosols) are
entirely different from those determining absorption of
gases.
8.1.5 Current Practice
8.1.5.1 Exhausts carrying acid compounds comprise
the largest volume of air discharged from a typical
semiconductor manufacturing plant.
8.1.5.2 The current practice is to connect most or all of
the corrosive sources to a central building exhaust
system and to discharge the combined gas streams to
one or more large wet scrubbers.
8.1.5.2.1 A variety of scrubber types might be used,
but the two main types are cross-flow and counter-
current scrubbers equipped with packing material of
some type.
8.1.5.3 The scrubbers usually are fed with a stream of
water (potable, recycled, etc.).
8.1.5.4 A side stream of water is bled from the
scrubber to a wastewater line.
8.1.5.5 It is futile to discharge organic solvents to
water scrubbers even if the organic solvents are, to
some degree, soluble in water.
8.1.5.5.1 Any dissolved organic solvent will eventually
be desorbed from the water and will escape to the
atmosphere, or be discharged as wastewater.
Additionally, these organics could serve to propagate
biological growth in the scrubber.
8.1.5.6 Exhausts from process tools are frequently
discharged to the appropriate building exhaust systems,
either with or without point-of-use abatement.
8.1.5.7 Arsine and phosphine, which have only slight
solubility in water, pass through the water scrubbers so
that the effect of discharging these gases into the
building scrubber system is limited to dilution.
Fluorine and chlorine also have limited solubility in
water and may pass through water scrubbers without
sufficient abatement. POU abatement devices should
be considered for these types of gases.
8.1.5.8 Preliminary oxidation of these and numerous
other process gases results in formation of fine fumes
and other aerosols that will be collected with only low
efficiencies by the typical large, low-energy wet
scrubbers.
8.1.5.9 If ammonia is discharged into a system that
also carries hydrogen chloride, hydrogen fluoride, nitric
acid, or sulfuric acid; it will react, in the vapor phase, to
SEMI F5-1101 © SEMI 1990, 2001 6
form sub-micron solid aerosols of ammonium chloride,
ammonium fluoride, ammonium nitrate, or ammonium
sulfate, respectively.
8.1.5.10 Any exhausts carrying ammonia should be
discharged to a separate scrubber or a point-of-use
ammonia abatement used at the process discharge.
8.1.6 Chemistry of Pollutants
8.1.6.1 Effective abatement of any compound requires
careful consideration of the chemistries and physical
states of the specific compounds being abated.
8.1.6.2 The packed-bed scrubbers commonly employed
should generally be adequate for absorption of readily
soluble gases, if appropriate operating conditions are
also maintained.
8.1.6.3 Misuse can negate the potential performance of
the scrubbers.
8.1.6.4 As scrubbers are low-energy devices (pressure
drops on the order of 2.5 to 10 cm (1-4 inches) of water,
they are incapable of effectively collecting fine aerosols
such as the ammonium salts mentioned above.
8.1.6.5 The most economic design approach, therefore,
is to arrange the exhaust system and use POU
abatement devices to avoid formation of such aerosols,
where possible.
8.1.6.6 The practice of using only recirculated water in
the scrubber, to absorb acid vapors and to neutralize the
water bleed stream should be reviewed to ensure that
scrubber efficiency is not affected.
8.1.6.7 Acid gases, such as hydrogen chloride and
hydrogen fluoride, are readily soluble in water. The
partial pressure of acid gases in equilibrium with the
solution, which increases with increasing solute
concentration, reduces the absorption of additional gas.
Reduction in absorption can be minimized by
increasing the rate of water blow-down (i.e., using more
fresh water make-up).
8.1.6.8 However, increasing the rate proportionately
increases the consumption of water and the volume of
water that must be treated before discharge as
wastewater.
8.1.6.9 The introduction of an alkaline solution (e.g.,
sodium hydroxide) in the scrubber can assist in
removing compounds from the gas stream if the use of
water alone does not provide adequate removal
efficiencies. The use of additives in the scrubber can, in
itself, create safety and environmental concerns.
8.1.6.10 Some of the neutralized absorbent must be
bled off from the recirculated stream to prevent the
brine concentration from reaching an excessively high
level, but water consumption and the volume of waste
can still be reduced by alkaline solution.
8.1.6.11 Aqua regia is a reactive mixture of nitric and
hydrochloric acids. The reaction produces nitrosyl
chloride and elemental chlorine:
HNO
3
+ 3HCl è NOCl + Cl
2
+ 2H
2
O
8.1.6.12 The nitrosyl chloride vapor is readily
decomposed by absorption in an alkaline solution:
NOCl + 2NaOH è NaNO
2
+ NaCl + H
2
O
8.1.6.13 Elemental chlorine has only a limited
solubility in water, and an alkaline solution (pH > 10)
must be used to attain adequate absorption efficiencies:
Cl
2
+ 2NaOH è NaOCl + NaCl + H
2
O
8.1.6.14 During the initial reaction of the nitric and
hydrochloric acids, hydrogen chloride gas and nitric
acid vapors are volatilized along with the nitrosyl
chloride and chlorine.
8.1.6.15 Nitrogen dioxide may also be produced by
decomposition of nitric acid.
8.1.6.16 All these compounds can be absorbed, to
some degree, in an alkaline solution.
8.1.6.17 If a pollutant is in a particulate form (solid or
liquid), its collection is only slightly affected by its
chemical nature or solubility.
8.1.6.18 The dominant factor determining particulate
collectability is particle size.
8.1.6.19 Abatement of particulate compounds requires
a high-energy method (usually creating an extremely
high pressure-drop across the abatement device) which
can be costly in both energy and abatement device size
requirements.
8.1.7 Scrubber Design and Selection
8.1.7.1 A variety of commercially available scrubbers
are applicable to absorption of readily soluble gases.
8.1.7.2 These include packed-bed counter-current, co-
current, and horizontal cross-flow scrubbers, horizontal
spray chambers, vertical spray towers, and gas-
atomizing scrubbers such as the venturi scrubber.
8.1.7.3 To some extent, the choice of scrubber type is a
matter of preference, as it is usually possible to obtain
equivalent performance on the soluble gases with
different types of devices; however, more packing or
higher liquid loading may be required on some devices.
8.1.8 The choice is likely to be limited by a
consideration of practical or economic factors such as
cost, available space, and weight (e.g., it is possible to